The dissection of electronic devices is a major source of information for both commercial competitors as well as foreign governments. In most instances, the devices are analyzed by techniques such as cross-sectioning and etching. The present invention relates to coatings comprising silica-containing matrices and certain fillers which inhibit this type of examination.
The use of silica-containing ceramic coatings on substrates such as electronic devices is known in the art. For instance, Haluska et al. in U.S. Pat. Nos. 4,749,631 and 4,756,977 disclose processes for forming silica coatings on electronic substrates wherein solutions of silica precursor resins are applied to substrates followed by heating the coated substrates in air at a temperature in the range of 200.degree.-1000.degree. C. These references, however, do not describe the use of fillers within the coating.
Similarly, the use of fillers within ceramic coatings is also known in the art. For instance, U.S. Pat. No. 3,986,997 describes a composition comprising an acidic dispersion of colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium which can be used to apply transparent abrasion resistant coatings on a variety of substrates. The reference, however, does not describe the use of the fillers described herein nor the application of a coating on an electronic substrate.
The present inventors have now discovered that tamper-proof coatings for electronic circuits can be formed from compositions comprising silica precursor resins and certain fillers.